The groundwater store is replenished by water draining from the lower soil zone store. The percentage of the contents of the lower soil zone store that drain to groundwater is a function of how close to full the lower zone store is.

The quantity of water draining from the lower zone, however, is independent of the status of the groundwater store and is obtained as follows:

 

where:

LDR = lower soil zone drainage

TF = time interval (days)

LDF = lower soil drainage factor